In general, imaging apparatuses using X-rays, ultrasound waves, and magnetic resonance imaging (MRI) are widely employed in the field of medicine. On the other hand, in the field of medicine, researches have been actively carried out to develop apparatuses employing an optical imaging technology in which light from a light source such as a laser is emitted to and propagated in a test object such as a living body, so that information in the living body is acquired by detecting the propagated light.
Photoacoustic Tomography (PAT) has been proposed as one of such optical imaging technologies. In PAT, pulsed light generated by a light source is emitted to a test object, whereby acoustic waves (hereafter, also referred to as photoacoustic waves) are generated by living body tissues which have absorbed optical energy propagated and diffused inside the test object. These photoacoustic waves are detected at a plurality of positions, and photoacoustic signals thus obtained are analyzed and processed so that information relating to optical characteristic values inside the test object can be visualized. This makes it possible to obtain an optical characteristic value distribution, particularly an optical energy absorption density distribution inside the test object with a high resolution.
Transducers utilizing piezoelectricity are typically employed as detectors of acoustic waves. Transducers utilizing changes in capacity are also being provided for general use.
In addition, a detector utilizing optical resonance has recently been studied and reported (see Non-Patent Literature 1). This known detector employs a technique in which acoustic waves are detected on the principle of Fabry-Perot interferometer (hereafter, also referred to as the FP method), and this detector is characterized by having broadband reception performance, providing high-definition images.
However, the FP method has a drawback of requiring long time for measurement. According to Non-Patent Literature 1, for example, in order to acquire two-dimensional distribution data of photoacoustic waves, a measurement light for evaluating optical reflectance is scanned by means of a galvanometer. This means that, in order to acquire one piece of volume data, optical resonance positions are raster scanned to acquire data at the respective positions. At the same time, in order to set an optimum wavelength at each of the measurement positions, the data are acquired while changing the measurement wavelength for each of the positions. It is reported that, according to this technique, it takes ten minutes or more to obtain a three-dimensional image of a few millimeters square.
In general, it is practically important for measurement equipment to acquire data in as short period of time as possible. In particular, when an object to be measured is a living body, the state of the test object is successively changed by effects of body motion or the like. Therefore, an adequate image cannot be obtained if it takes long time to acquire data.
An attempt has been reported in which in order to collectively acquire two-dimensional distribution of elastic waves, an acoustic pressure of ultrasound waves acquired by a FP-type reception element is detected by using a CCD camera as a two-dimensional array sensor (see Non-Patent Literature 2).